1. Field of the Invention
The present invention relates to variable discharge high pressure pumps for supplying metered quantities of fuel to a common rail of a diesel engine.
2. Description of Related Art
Conventional variable displacement high pressure pumps typically have a plurality of pumping elements, each of which comprises a pumping chamber in which a pumping plunger is reciprocated by a rotary cam, and to which fuel is supplied at low pressure (approximately 40 psi.) by a low pressure pump. Examples of such high pressure pumps can be found in, e.g., U.S. Pat. Nos. 5,133,645; 5,094,216; 5,058,553, 4,777,921 and 4,502,445.
Furthermore, usually, a high pressure pump will have two to four pumping elements, depending on pumping capacity, and a respective solenoid valve is used to control the quantity of fuel metered into each of the pumping units. For cost and other reasons, it is desirable to enable metering of fuel into the pumping chambers of a plurality of pumping units to be controlled by no more than a single solenoid valve.
In operation, conventional variable displacement high pressure pumps maintain the solenoid valves in a normally open position and fuel flows into and fills the pumping chambers during the retraction stroke of the pumping plunger. When the pumping plunger starts its compression stroke, fuel spills through the open solenoid valve until it receives a command signal to close. At that point, the fuel remaining in the pumping chamber is trapped and pressurized by the pumping plunger which causes the fuel to flow to the common rail at high pressure. Since the solenoids are caused to close during the compression stroke of the pumping plunger, they must act against the high pressure (15 kpsi. or higher) of the spilling fuel to seal. Thus, for durability and cost reasons, there is a need for a high pressure variable displacement pump which can use solenoid valves which need only be able to act in a low pressure range (e.g., about 40 to 100 psi.).
U.S. Pat. Nos. 5,109,822 and 5,035,221, disclose high pressure common rail fuel injection systems for diesel engines in which a pair of pumping elements is controlled by the same solenoid valve. However, both of the pumping elements of the pair that is controlled by the same solenoid valve are filled and discharged in unison, and to enable fuel to be supplied to the common rail when that pair of pumping elements is being filled, a second pair of pumping elements is provided which is controlled by a second solenoid valve. Thus, it is deskable to be achieve a manner of controlling a plurality of pumping elements via a single solenoid which would enable the pumping elements to be supplied with fuel at different times.
Metering of fuel can be controlled in accordance with a number of different techniques, such as (1) time/stroke metered (TS), (2) time metered at a constant pressure (TP), and (3) pressure metered at a constant time interval (PT). Normally, a fuel injection system is designed to operate under only a given one of these or other metering principles, with cost and size constraints governing the selection more than any other factor. As such, it would be advantageous to have a variable displacement high pressure pump which is readily adaptable to operation in accordance with any of the noted, TS, TP, and PT, metering techniques.